Make pkgConfigExecutable() work when a pkg-config is defined in the spec

Commit 6e6f27b6 made it possible to set the PKG_CONFIG variable using
CROSS_COMPILE as a prefix. The problem with that solution is that it makes
pkgConfigExecutable() skip the environment setup for pkg-config as well,
as it expects the pre-set command to be self-contained - which it isn't.
To avoid this problem we need to store the pkg-config define in the
device spec in a separate variable.

Change-Id: Id8ae7fb03d9253be55840e23fe73b30815ee86c3
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/devices/common/linux_device_pre.conf

@@ -25,4 +25,4 @@ QMAKE_NM                = $${CROSS_COMPILE}nm -P
 QMAKE_STRIP             = $${CROSS_COMPILE}strip
 
 contains(DISTRO_OPTS, deb-multi-arch): \
-    PKG_CONFIG = $${CROSS_COMPILE}pkg-config
+    QMAKE_PKG_CONFIG = $${CROSS_COMPILE}pkg-config

mkspecs/features/qt_functions.prf

@@ -215,7 +215,10 @@ defineTest(qtAddTargetEnv) {
 
 defineReplace(pkgConfigExecutable) {
     isEmpty(PKG_CONFIG) {
-        PKG_CONFIG = pkg-config
+        !isEmpty(QMAKE_PKG_CONFIG): \
+            PKG_CONFIG = $$QMAKE_PKG_CONFIG
+        else: \
+            PKG_CONFIG = pkg-config
 
         sysroot.name = PKG_CONFIG_SYSROOT_DIR
         sysroot.value = $$PKG_CONFIG_SYSROOT_DIR